In commonly used electric storage batteries, such as the well known 12-volt battery employed in cars, it has been a desiratum to have a battery separator between the battery plates as thin as is possible to have the lowest possible electrical resistance. At the same time, it has been sought to obtain a battery separator which is reasonably flexible and yet does not develop failure in use.
Generally, a battery separator is needed as a spacer and to prevent two plates from touching each other causing a short. At the same time, a separator shall not impede the electrolyte flow. Also, a fine pore size is desirable to prevent dendrite growth developing between adjacent plates. The result of dendrite growth is a battery "short". It has been necessary not only to increase the battery plate spacing, but also to use battery separators to prevent a battery "short" for one or more of the reasons given above.
Various other problems have also resulted from spalling of the battery plates associated with the use of antimony or calcium additives to lead plates. Spalled deposits at the bottom of the battery have likewise caused shorts or premature failure of the battery. For this reason, it has been sought to have a battery which could be made in a manner whereby the battery separators could envelop or be festooned around the plates or made in a serpentine fashion thereby isolating one plate from the other.
However, the prior art battery separators have been invariably rather stiff and inflexible; complex shapes could only be formed with great difficulty.
Better quality batteries have, as separators, cured natural rubber compositions. A common disadvantage inherent in the use of rubber or natural rubber based battery separators is that a sulfur cure process produces rather stiff and brittle separators. Further, a sulfur cure process is capital intensive requiring compounding mixers, milling equipment, extruders, a battery of vulcanizers, etc. Sulfur curing of rubber battery separators is a batch process. A batch process is also labor and energy intensive. Still further, in order to maintain the desired porosity in a rubber battery separator provided by rehydrated silica, the separator must be sulfur cured in a water filled autoclave. Repeated raising and lowering the temperature of large amounts of water is very energy consuming.
In curing of the rubber composition, the cured articles are tested for cracking and brittleness. Unless very careful processing steps are followed in making sulfur cured separators, problems of brittle cracking often result.